Does the Stein panel replace the entire plans based panel structure? If the panel is flexing from just the rudder trim, I would think it needs to be better supported. Imagine what would happen in an accident where there are much stronger forces than two small springs.
Most panels, including Aerosport Products, uses the plans based panel sub-structure and only replace the one piece of aluminum that Van's intended to be the panel.
I would think that the carbon fiber would flex more that aluminum, unless the aluminum is pretty thin. Mine is installed on the comm stack insert on the standard panel from Aerosport Products. I don't experience any flexing from the panel.